The present invention relates to an element using a Coulomb-blockade phenomenon and formed on a silicon substrate and a method of manufacturing the same.
A Coulomb-blockade phenomenon in electron tunneling at a small tunnel junction is a phenomenon in which tunneling of one electron is suppressed as free energy based on charging energy accompanying the tunneling increases. A Coulomb-blockade element using such a Coulomb-blockade phenomenon can control currents and charges flowing out from or stored in the element in units of electrons. For this reason, the power consumption per element is very small. In addition, the device area is also very small. Owing to these characteristics, an integration degree much higher than the integration limits of the existing silicon-based integrated circuits is expected. As the basic structure of this element, a single electron transistor or a single electron memory has been proposed.
Most of conventional Coulomb-blockade elements have a structure in which electrons are confined in a small metal island by using a small tunnel junction of a metal/metal oxide or a structure in which a two-dimensional electron gas formed at a heterojunction of a III-V compound semiconductor is confined in the form of an island by using an electric field generated by a narrow electrode or the like formed on the junction.
FIG. 31 shows the conventional Coulomb-blockade element disclosed in "Single-Electron Charging and Periodic Conductance Resonances in GaAs Nanostructures", U. Meirav et. al., Phys. Rev. Lett., Vol. 65, No. 6, pp. 771-774, 1990. FIG. 32 shows an equivalent circuit of this Coulomb-blockade element. Reference numeral 71 denotes a substrate consisting of n-type GaAs; 72, an AlGaAs layer; 73, a GaAs layer; and 74, an electrode formed on the GaAs layer 73.
In such a Coulomb-blockade element, a two-dimensional electrode gas is formed at the heterointerface between the AlGaAs layer 72 and the GaAs layer 73. Constrictions 75 which are constricted in the horizontal direction are formed on the electrode 74 to form potential barriers. A region 76 between these barriers becomes a conductive island for confining charges.
The potential barrier between the conductive island 76 and a source electrode 77 serves as a tunnel capacitance Cs. The potential barrier between the conductive island 76 and a drain electrode 78 serves as a tunnel capacitance Cd. As a result, an element having an equivalent circuit like the one shown in FIG. 32 is formed.
One of the most important subjects in putting such a Coulomb-blockade element to practical use is an operating temperature. In order to operate the Coulomb-blockade element at a practical temperature, a conductive island serving as an electron reservoir, which is the core of the element, must be formed on the nm scale, and a tunnel barrier having a very small capacitance of several aF (1 aF=10.sup.-18 F) must be formed. This is because the charging energy of a single electron is buried in thermal energy as the sizes of these portions increase, and no Coulomb-blockade phenomenon can be observed.
Another important subject is how to manufacture and arrange such minute structures with good controllability. In order to realize a new function by coupling Coulomb-blockade elements, in particular, a manufacturing technique with good controllability is required.
In the Coulomb-blockade element shown in FIG. 31, the width (in the lateral direction in FIG. 31) of each constriction 75, where the gap in the electrode 74 is the smallest, must be sufficiently smaller than the width of the conductive island 76. For this reason, if the constriction 75 is to be manufactured by electron beam lithography, the size of the island 76 inevitably becomes much larger than the minimum dimensions determined by the lithography limit.
With regard to a Coulomb-blockade using a metal, since no effective working technique of forming a metal island is available, it is difficult to form a small metal island with good reproducibility.
Either of the elements having the above structures, therefore, can operate only at a very low temperature of 1 K or less.
A method of forming a conductive island by using fluctuations of a structure made of a thin polycrystalline material is known as an effective means for attaining a small capacitance (K. Yano et al., "Room-Temperature Single-Electron Memory", IEEE Trans. Electron Devices, Vol. 41, p. 1628, 1994). In this method, however, the position and the size of a conductive island cannot be arbitrarily controlled because fluctuations caused by a polycrystalline structure are used.
As described above, in the conventional methods, a technique of manufacturing a very small conductive island and a technique of positioning the conductive island with good reproducibility have not been established. These techniques are indispensable for putting a Coulomb-blockade element to practical use. Therefore, a Coulomb-blockade element which can operate at room temperature cannot be realized with good controllability and good reproducibility.